1. Field of the Invention
The present invention relates to a document processing apparatus with a function of scanning an image shown on a document such as an original and a method of scanning the image. More particularly, the present invention relates to a document processing apparatus with image scanning resolution correction function and a method of correcting image scanning resolution.
2. Description of the Related Art
A document processing apparatus with a function of scanning an image shown on a document is an arbitrary appropriate document processing apparatus in this technical field. This type of document processing apparatuses include, for example, a scanner apparatus, a copier, a facsimile apparatus, and a multi-function peripheral that that has two or more functions among functions of these apparatuses.
A document processing apparatus of an scanning scheme for a placed original on the glass with a function of scanning an image shown on a document includes a light source that irradiates light on an original placed on an original placing glass plate, on which an original is placed, three mirrors that guide reflected light of the light irradiated on the original in a predetermined direction, and a lens to which output light from a last mirror among these mirrors is input. A portion of the original illuminated by the light source is imaged on a light receiving surface of an optical sensor such as a CCD by the lens. Image data converted into an electric signal is outputted from the optical sensor. Usually, among the three mirrors, a first mirror that first receives reflected light of the light irradiated on the original is provided in a first carriage. Both second and third mirrors are provided in a second carriage. Further, usually, the light source is provided in the first carriage.
In this type of the document processing apparatus with the function of scanning an image shown on a document, a sum of an optical path length from an upper surface of the original placing glass plate, on which an original is placed, i.e., an original surface as a scanning object to a reflecting point of the first mirror, an optical path length from the reflecting point of the first mirror to a reflecting point of the second mirror, an optical path length from the reflecting point of the second mirror to a reflecting point of the third mirror, and an optical path length from the reflecting point of the third mirror to the lens needs to be a value within a predetermined range. This is for the purpose of securing predetermined resolution and keeping image magnification substantially constant.
For example, when a document is placed on the original placing glass plate, on which an original is placed, to scan the document, the first carriage is moved to scan the document. In this case, when the second carriage remains stationary, according to the movement of the first carriage, the optical path length from the reflecting point of the first mirror to the reflecting point of the second mirror and the optical path length from the reflecting point of the third mirror to the lens change. Therefore, the sum of the optical path lengths changes. When the sum of the optical path lengths changes, an image on the light receiving surface of the optical sensor is not appropriately imaged to cause so-called out-of-focus. As a result, the predetermined resolution of the document processing apparatus with the function of scanning an image shown on a document may not be secured.
To prevent such a situation, when scanning is performed using the first carriage to scan the document, to keep the sum of the optical path lengths constant, the second carriage is moved in parallel to and in an identical direction with the first carriage at scanning speed a half of scanning speed of the first carriage. To realize this, a mechanism described below is disclosed in Jpn. Pat. Publication (Kokai) No. H7-140565. The first carriage and the second carriage are driven by one driving motor via a driving pulley and move. The driving pulley includes a combination of large and small two pulleys that coaxially have a diameter ratio of 1:2. The first carriage is driven via the pulley with the large diameter and the second carriage is driven via the pulley with the small diameter. By adopting such a mechanism, it is possible to set a ratio of moving speed of the first carriage and moving speed of the second carriage as 2:1.
However, when a member deciding a moving direction of the carriages, for example, carriage sliding rails, and the original placing glass plate, on which an original is placed, are not parallel, the moving direction of the carriages and the original are not parallel. Thus, the sum of the optical path length is not kept constant even by the mechanism for moving the second carriage at scanning speed a half of scanning speed of the first carriage. When a dimension error of a member constituting a part of the document processing apparatus is large, the same situation occurs. As a result, there is a problem in that it is impossible to secure the predetermined resolution and keep image magnification constant.
Other than the scanning scheme for a placed original on the glass for scanning a document by moving the carriages in a state in which an original is placed stationary on the original placing glass plate, there is a system for scanning a document by conveying an original in a state in which the two carriages are kept stationary. The latter system is called an scanning scheme for fed original and is sometimes called a sheet-through scanning system by those having ordinary skill in the art. Usually, an original is conveyed by a document feeder. In the scanning scheme for fed original, it is possible in principle to scan an original, which passes over a glass plate, at predetermined resolution by arranging two carriages in appropriate positions. Usually, the positions of the two carriages are decided to scan the original in a place substantially in contact with the upper surface of the glass plate. However, for example, when a document as a scanning object is conveyed apart from the glass surface, it may be impossible to scan the document at the predetermined resolution.
As described above, scanning resolution as designed may not be obtained because of a dimension error of members constituting the document processing apparatus, an assembly error in assembling the apparatus, or the like. Recently, there is an increasing need for a document processing apparatus with excellent scanning resolution. However, member with high accuracy and skilled manufacturing techniques are necessary to embody the document processing apparatus with high resolution. This causes an increase in cost.
Therefore, a document processing apparatus with which predetermined scanning resolution is obtained even if there are some dimension errors of members, assembly errors, and the like is desired.